Apparatus and method for selectively coupling system with a first power supply or a second power supply

ABSTRACT

An apparatus for selectively coupling a system with a first power supply or a second power supply includes: (a) a first switch for effecting a first coupling of the system with the first power supply; (b) a second switch for effecting a second coupling of the system with the second power supply; (c) a first switch control unit coupled for controlling the first switch; (d) a second switch control unit coupled for controlling the second switch; (e) a connection director unit coupled with the first and second switch control units for providing control signals to effect the first and second coupling; at least one of the first and second coupling being effected as an initial coupling establishing a generally time-dependent increasing current between the system and one power supply and a continuing coupling establishing a substantially constant operating current between the system and the one power supply.

BACKGROUND OF THE INVENTION

The present invention is directed to systems having a plurality of powersupplies. A preferred embodiment of the present invention is directed tosystems that have a battery power supply and an alternating currentpower supply.

Two problems may occur when switching power supplies such as, by way ofexample and not by way of limitation, when switching between batterypower supply and a dc power supply such as would be derived from an ACadapter for a notebook computer or a handheld computing or communicationdevice. First, when powering up the system using an AC adapter powersupply, high peak currents (e.g., in excess of 70 Amps) may occur. Sucha condition may cause reliability problems and usually adds to the costof the system because of increased robustness of components required towithstand the rigors of high inrush current such as, by way of exampleand not by way of limitation, increased robustness of protectioncomponents and switches. Second, there can be reliability issuesrelating to switching between power supplies that have voltages higherthan the ratings for commercially available PMOS (P-channel Metal OxideSemiconductor) power switches (i.e., approximately 20 volts). Circuitdesigners commonly have added protection circuits to overcome thisreliability issue. However, the protection circuitry may sometimeseffect a lock up condition when switching between power supplies at highvoltage.

There is a need for an apparatus and method that can effect switchingbetween power supplies that limits current inrush when powering up asystem using an AC adapter-derived power supply.

There is a need for an apparatus and method that can effect switchingbetween power supplies that avoids a lock up condition when switchingbetween power supplies.

SUMMARY OF THE INVENTION

An apparatus for selectively coupling a system with a first power supplyor a second power supply includes: (a) a first switch for effecting afirst coupling of the system with the first power supply; (b) a secondswitch for effecting a second coupling of the system with the secondpower supply; (c) a first switch control unit coupled for controllingthe first switch; (d) a second switch control unit coupled forcontrolling the second switch; (e) a connection director unit coupledwith the first and second switch control units for providing controlsignals to effect the first and second coupling; at least one of thefirst and second coupling being effected as an initial couplingestablishing a generally time-dependent increasing current between thesystem and one power supply and a continuing coupling establishing asubstantially constant operating current between the system and the onepower supply.

A method for selectively coupling a system with a first power supply ora second power supply includes the steps of: (a) in no particular order:(1) providing a first switch connected for switchingly effecting a firstcoupling of the system with the first power supply; (2) providing asecond switch connected for switchingly effecting a second coupling ofthe system with the second power supply; (3) providing a first switchcontrol unit coupled with the first switch for controlling the firstswitch; (4) providing a second switch control unit coupled with thesecond switch for controlling the second switch; and (5) providing aconnection director unit coupled with the first switch control unit andcoupled with the second switch control unit; (b) operating theconnection director unit to provide a first plurality of control signalsand to provide a second plurality of control signals; (c) operating thefirst switch control unit and the second switch control unit to respondto the first plurality of control signals and the second plurality ofcontrol signals to selectively effect the first coupling and the secondcoupling; and (d) effecting at least one of the first coupling and thesecond coupling as an initial coupling and a subsequent continuingcoupling; the initial coupling establishing a generally time-dependentincreasing current between the system and one power supply of the firstpower supply and the second power supply; the continuing couplingestablishing a substantially constant operating current between thesystem and the one power supply.

It is, therefore, an object of the present invention to provide anapparatus and method that can effect switching between power suppliesthat limits current inrush when powering up a system using an AC adapterpower supply.

It is another object of the present invention to provide an apparatusand method that can effect switching between power supplies that avoidsa lock up condition when switching between power supplies.

Further objects and features of the present invention will be apparentfrom the following specification and claims when considered inconnection with the accompanying drawings, in which like elements arelabeled using like reference numerals in the various figures,illustrating the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical schematic diagram illustrating a prior artapparatus for selectively coupling a system with a first power supply ora second power supply.

FIG. 2 is an electrical schematic diagram illustrating an apparatus forselectively coupling a system with a first power supply or a secondpower supply configured according to the present invention.

FIG. 3 is an electrical schematic diagram illustrating an alternateembodiment for the timer unit of the apparatus of the present invention.

FIG. 4 is a flow diagram illustrating the method of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an electrical schematic diagram illustrating a prior artapparatus for selectively coupling a system with a first power supply ora second power supply. In FIG. 1, an apparatus 10 is configured forselecting a power supply for a dual supply system (not shown in FIG. 1)coupled with a system connection locus 18. Apparatus 10 includes aconnection director unit 12 coupled with switching units 14, 16.

Switching unit 14 includes a switch control unit 20 coupled with a PMOSswitch 22. Switch control unit 20 includes an AND gate 24 and an“inverter” 26 (“inverter” 26 is a level shifter which translates adigital control signal to a level-shifted digital signal, ACDRV, the newlevel-shifted signal having a high voltage of V_(CC), for example, and alow voltage of V_(CC)-V_(NEG)). V_(NEG) is a predetermined lower supplyvoltage established at a level below supply voltage V_(CC). Thelevel-shifting inverter 26 will hereafter be called simply an inverter.AND gate 24 receives a first input 30 (signal ACON; AC adapter voltageconnection on) from connection director unit 12 and receives a secondinput 32 from switching unit 16. When signal ACON received at firstinput 30 is a “1” and the signal received from switching unit 16 atsecond input 32 is a “1”, AND gate 24 provides a “1” signal to inverter26. When inverter 26 receives a “1” from AND gate 24, inverter 26presents a signal ACDRV as a “0” (V_(CC)-V_(NEG)) to PMOS switch 22,thereby turning on PMOS switch 22 and coupling system connection locus18 with an AC adapter power supply (not shown in FIG. 1) coupled at anAC adapter power input locus 36. Signal ACDRV is also provided to acomparator 40. Comparator 40 receives a voltage of (V_(CC)-V_(REF);V_(REF) is a reference voltage) at an inverting input locus 42. Whensignal ACDRV is “0”, comparator 40 presents a “0” at an output locus 46.When signal ACDRV is “1” (i.e., at voltage level V_(CC)), comparator 40presents a “1” at output locus 46.

Switching unit 16 includes a switch control unit 50 coupled with a PMOSswitch 52. Switch control unit 50 includes an AND gate 54 and aninverter 56. AND gate 54 receives a first input 60 (signal BATON;battery connected to system) from connection director unit 12 andreceives a second input 62 from switching unit 14. When signal BATONreceived at first input 60 is a “1” and the signal received fromswitching unit 14 at second input 62 is a “1”, AND gate 54 provides a“1” signal to level-shifting inverter 56. Level-shifting inverter 56will hereafter be called simply an inverter. When inverter 56 receives a“1” from AND gate 54, inverter 56 presents a signal BATDRV as a “0” toPMOS switch 52, thereby turning on PMOS switch 22 and coupling systemconnection locus 18 with a PACK (i.e., battery pack) power supply (notshown in FIG. 1) coupled at a PACK power input locus 66. Signal BATDRVsignal is also provided to a comparator 70. Comparator 70 receives areference voltage, V_(CC)-V_(REF), at an inverting input locus 72. Whensignal BATDRV is “0”, comparator 70 presents a “0” at an output locus76. When signal BATDRV is “1”, comparator 70 presents a “1” at outputlocus 76.

Output 46 is coupled with input locus 62 of AND gate 54 so that a “1”cannot be provided to inverter 56 whenever signal ACDRV is a “0”. Bythis arrangement signal BAT DRV cannot close PMOS switch 52 when PMOSswitch 22 is closed. Output 76 is coupled with input locus 32 of ANDgate 34 so that a “1” cannot be provided to inverter 36 whenever signalBATDRV is a “0”. By this arrangement signal ACDRV cannot close PMOSswitch 22 when PMOS switch 52 is closed.

Switching unit 16 also includes a protection circuit 80 that includesZener diodes 82, 84 opposingly coupled between source 53 and gate 55 ofPMOS switch 52. Protection circuit 80 keeps V_(GS) (gate-to-sourcevoltage) of PMOS switch 52 from exceeding a predetermined value such as,by way of example and not by way of limitation, 20 volts, absolute(i.e., ±20 volts).

Using apparatus 10, when either PACK power input locus 66 or AC adapterpower input locus 36 is switched to couple with system connection locus18 in response to signals ACON, BATON from connection directing unit 12,comparators 40, 70 operate as “break-before-make” systems. Comparator 70controls operation of AND gate 24 to ensure that signal BATDRV no longerturns on PMOS switch 52 before signal ACDRV can turn on PMOS switch 22.Comparator 40 controls operation of AND gate 54 to ensure that signalACDRV no longer turns on PMOS switch 22 before signal BATDRV can turn onPMOS switch 52.

During power up conditions using an AC adapter power supply unit (notshown in FIG. 1) connected with AC adapter power input locus 36 with nobattery pack present (or with battery pack present), the secondarycircuit of the AC adapter unit (not shown in FIG. 1) can provide veryhigh current to connection locus 18 for short periods of time. Becausethe AC adapter is already on with power at locus 36, very high currentpeaks are often present in PMOS switch 22. It is difficult to controlhow fast the (internal or external) PMOS switch 22 connects the ACadapter. By way of example and not by way of limitation, with notebookcomputers having a 5 amp standard AC adapter unit coupled, peak currentsfrom the AC adapter unit to the notebook computer (i.e., the systemcoupled with system connection locus 18) can reach 70 Amps for severalhundred microseconds until protection circuitry internal to the ACadapter unit is activated. Such potential for a high current gives riseto reliability considerations in designing systems because of powerdissipation (and associated heat) and excessive current through PMOSswitch 22. Decreasing rise times at gate 25 of PMOS switch 22 reducesthe undesirable current peak but at the expense of additional powerdissipation and heat, and is therefore not an effective solution to thiscurrent inrush problem.

Another drawback with apparatus 10 is that PMOS 52 must always be turnedoff using the higher voltage applied at system connection locus 18. Manyexternal switching FET (Field Effect Transistor) devices having a ratedV_(GS) at 20 volts. This high voltage requires using protection circuit80 (described above) to clamp gate 55 of PMOS switch 52 during turn-offtransients. For systems operating with voltages higher than the clampingvoltage of protection circuit 80, a lockup condition will occur when acommand to switch to AC adapter power is issued by connection directingunit 12 and BATDRV locus 55 exceeds the system voltage at systemconnection locus 18 by more than a predetermined amount, such as 20volts. That is, when connection directing unit 12 changes signal ACONfrom “0” to “1” and changes signal BATON from “1” to “0” and voltage atBATDRV input locus 55 exceeds voltage at system connection locus 18 by20 volts or more (for example), Zener diodes 82, 84 (gate clamp diodes)will break down when signal BATDRV increases (BATDRV “1” is typicallyset to voltage V_(CC). Voltage at system, locus 18, can initially be aslow as 0 V). Under such conditions, the threshold voltage atnoninverting input locus 74 of amplifier 70 will never be reached,amplifier 70 will not fulfill its break-before-make function, PMOSswitch 52 will be off while PMOS switch 22 will also be left offindefinitely, thereby leaving the system locus 18 unpoweredindefinitely, even though power is desired from the AC adapter.

FIG. 2 is an electrical schematic diagram illustrating an apparatus forselectively coupling a system with a first power supply or a secondpower supply configured according to the present invention. In FIG. 2,an apparatus 110 is configured for selecting a power supply for a dualsupply system (not shown in FIG. 2) coupled with a system connectionlocus 118. Apparatus 110 includes a connection director unit 112 coupledwith switching units 114, 116.

Switching unit 114 includes a switch control unit 120 coupled with aPMOS switch 122. Switch control unit 120 includes an AND gate 124 and aninverting level-shifter (hereafter referred to as an inverter) 126. ANDgate 124 receives a first input 130 (signal ACON; AC adapter-to-systemon) and a second signal SOFT (soft start) from connection director unit112. AND gate 124 receives a second input 132 from switching unit 116.

Preferably, AC adapter power is initiated for apparatus 110 using signalSOFT for a predetermined time to initiate a soft start for apparatus110. To initiate a soft start, signal SOFT is set to a “1”. When thesignal received from switching unit 116 at input 132 is also a “1” (atthe same time signal SOFT is set to a “1”) inverter 126 is set to a HIGHZ state in which output signals from inverter 126 do not seek to drivetoward a maximum value or toward a minimum value. In this HIGH Zorientation, a soft start circuit 190 is actuated and serves toestablish a generally time-dependent increasing current through a PMOStransistor 122. Signal SOFT turns on an NMOS (N channel Metal OxideSemiconductor) switch 191 to complete a circuit through an RC(Resistor-Capacitor) circuit including a resistor 192 coupled between aV_(NEG) voltage node 127 and a locus 193 in common with a gate 125 ofPMOS transistor 122. Soft start circuit 190 also includes a capacitor194 coupled between locus 193 and drain 123 of PMOS transistor 122,which is in common with a system connection locus 118 to which thesystem to be powered is coupled (the system is not shown in FIG. 2).Applying signal SOFT to soft start circuit 190 permits a relativelygradual increase of gate current applied at gate 125, therebyestablishing a generally time-dependent increasing current betweensystem connection locus 118 and an AC power input locus 136 to which anAC power supply (not shown in FIG. 2) is coupled. High levels of inrushcurrent are thereby avoided. After a time, signal SOFT is set to “0”,turning off NMOS transistor 191, and setting signal ACON at first input130 to “1”. When signal ACON is a “1” and the signal received fromswitching unit 116 at second input 132 is a “1”, AND gate 124 provides a“1” signal to inverter 126. When inverter 126 receives a “1” from ANDgate 124, inverter 126 presents a signal ACDRV as a “0” to PMOS switch122, thereby turning on PMOS switch 122 and maintaining coupling betweensystem connection locus 118 and an AC adapter power supply (not shown inFIG. 2) coupled at AC adapter power input locus 136. Signal ACDRV isalso provided to a comparator 140. Comparator 140 receives a referencevoltage, V_(CC)-V_(REF), at an inverting input locus 142. When signalACDRV is “0”, comparator 140 presents a “0” at an output locus 146. Whensignal ACDRV is “1”, comparator 140 presents a “1” at output locus 146.

Switching unit 116 includes a switch control unit 150 coupled with aPMOS switch 152. Switch control unit 150 includes an AND gate 154 and aninverter 156. AND gate 154 receives a first input 160 (signal BATON;battery switch 152 on) from connection director unit 112 and receives asecond input 162 from switching unit 114. When signal BATON received atfirst input 160 is a “1” and the signal received from switching unit 114at second input 162 is a “1”, AND gate 154 provides a “1” signal toinverting level-shifter (hereafter referred to as an inverter) 156. Wheninverter 156 receives a “1” from AND gate 154, inverter 156 presents asignal BATDRV as a “0” to PMOS switch 152, thereby turning on PMOSswitch 152 and coupling system connection locus 118 with a PACK (i.e.,battery pack) power supply (not shown in FIG. 2) coupled at a PACK powerinput locus 166. Signal BATDRV is also provided to comparator 170.Comparator 170 receives a reference voltage of V_(CC)-V_(REF) at aninverting input locus 172. When signal BATDRV is “0”, comparator 170presents a “0” at an output locus 176. When signal BATDRV is “1”,comparator 170 presents a “1” at output locus 176.

Output 146 is coupled with input locus 162 of AND gate 154 so that a “1”cannot be provided to inverter 156 whenever signal ACDRV is a “0”. Bythis arrangement signal BATDRV cannot close PMOS switch 152 when PMOSswitch 122 is closed.

Switching unit 116 also includes a protection circuit 180 that includesZener diodes 182, 184 opposingly coupled between source 153 and gate 155of PMOS switch 152. Protection circuit 180 keeps V_(GS) (gate-to-sourcevoltage) of PMOS switch 152 from exceeding a predetermined value suchas, by way of example and not by way of limitation, 20 volts.

Output 176 is coupled with a reset node 204 of a timer unit 200. Timerunit 200 receives signal BATON at an inverting set node 202. Timer 200presents output signals at an output node 206. An OR gate 208 has afirst input 210 coupled with output 176 and a second input 212 coupledwith timer output node 206.

When signal BATON goes to “0” (and signal ACON goes to “1”) indicatingan order from connection director unit to disconnect system locus 118from PACK power input locus 136 and to connect system connection locus118 with AC power input locus 136, signal BATON is applied at invertingset node 202, thereby setting timer unit 200. If voltage at BATDRV inputlocus 155 exceeds voltage at system connection locus 118 by 20 volts ormore, Zener diodes 182, 184 (gate clamp diodes) will break down whensignal BATDRV increases, as described earlier herein in connection withFIG. 1. However, OR gate 208 will apply a “1” signal if either of itsinput loci 210, 212 experiences a “1” signal. That is, if amplifier 170reaches sufficient threshold voltage to present a “1” signal at output176, OR gate 208 will present a “1” to AND gate 124 on line 132, therebypermitting AND gate 124 to respond to signal ACON going to a “1” valueby presenting a “1” to inverter 126 and gating PMOS transistor 122 tocomplete a circuit between system connection locus 118 and AC powerinput locus 136. A “1” signal on line 176 will also be applied to resetnode 204 of timer unit 200 and reset timer unit 200. This is a normalshift from battery power to AC power.

If a lock up condition occurs with amplifier 170 (as described earlierherein with in connection with FIG. 1), then timer unit 200 will ensurethat a “1” signal is presented from output node 206 to input 212 of ORgate 208 after a predetermined time has elapsed, so that PMOS switch 122will be turned on by switch control unit 120 substantially as describedwhen output 176 is a “1”. This use of timer unit 200 precludes therebeing an indefinite period for a lock up condition being experienced bybreak-before-make circuit (embodied in amplifier 170). Breakdown ofZener diodes 182, 184 and therefore possible indefinite turn off of bothPMOS transistor 152 and PMOS transistor 122 is thereby avoided (anindefinite state where there is no system power for an indefinitely longtime is avoided). The make-before-break configuration ensures that PMOSswitch 152 is turned off before signal ACON can effect turning on ofPMOS switch 122. It is preferable that apparatus 110 effect initialpowering up using AC adapter power and effect any change to AC adapterpower from battery power (or from no initial system power) using aninitial condition with signal SOFT at a “1” followed by a continuingcondition in which signal ACON is at a “1”.

FIG. 3 is an electrical schematic diagram illustrating an alternateembodiment for the timer unit of the apparatus of the present invention.In FIG. 3, line 376 from a break-before-make circuit embodied in acomparator (not shown in FIG. 3) is coupled with a reset node 404 of atimer unit 400. Timer unit 400 receives signal BATON at an inverting setnode 402. Timer unit 400 presents output signals at an output node 406.An OR gate 408 has a first input 410 coupled with output 376 and asecond input 412 coupled with timer output node 406.

When signal BATON goes to “0” (and signal ACON goes to “1”) indicatingan order from a connection director unit (not shown in FIG. 3) todisconnect a system (not shown in FIG. 3) from battery power and toconnect the system with AC power, signal BATON is applied at invertingset node 402, thereby setting timer unit 400. If battery BATDRV voltagewith respect to system voltage is sufficiently high, protection circuitsprotecting transistor switching units will break down, as describedearlier herein in connection with FIGS. 1 and 2. OR gate 408 will applya “1” signal if either of its input loci 410, 412 experiences a “1”signal. If sufficient threshold voltage is present to support normaloperations, line 376 will present a “1” at input 410 and OR gate 408will present a “1” to AND gate 324 on line 332, thereby permitting ANDgate 324 to respond to signal SOFT going to a “1” value by presenting a“1” to inverting level-shifter (hereafter referred to simply asinverter) 326 to set inverter 326 in a HIGH Z mode. Only when signalSOFT is “1” and output from OR gate 408 is “1” and signal ACON is “1”can inverter 326 turn on PMOS transistor 122 to complete a circuitbetween a system and AC power.

If a lock up condition occurs because of not achieving a thresholdvoltage (as described earlier herein with in connection with FIGS. 1 and2), then timer unit 400 will ensure that a “1” signal is presented fromoutput node 406 to input 412 of OR gate 408 after a predetermined timehas elapsed, so that PMOS switch 322 will be turned on substantially asdescribed when output 376 is a “1”. This use of timer 400 precludesthere being an indefinite period for a lock up condition beingexperienced by break-before-make circuit (embodied in comparator 370;not shown). Breakdown of protective circuits and damage to PMOS switchesare thereby avoided. The make-before-break configuration of the timerunit configuration illustrated in FIG. 3 ensures that a PMOS switchconnecting a system with battery power is turned off before eithersignal SOFT or signal ACON can effect connecting a system with AC power.

FIG. 4 is a flow diagram illustrating the method of the presentinvention. In FIG. 4, a method 500 for selectively coupling a systemwith a first power supply or a second power supply begins at a STARTlocus 502. Method 500 continues with the step of, in no particularorder, (1) providing a first switch connected for switchingly effectinga first coupling of the system with the first power supply, as indicatedby a block 504; (2) providing a second switch connected for switchinglyeffecting a second coupling of the system with the second power supply,as indicated by a block 506; (3) providing a first switch control unitcoupled with the first switch for controlling the first switch, asindicated by a block 508; (4) providing a second switch control unitcoupled with the second switch for controlling the second switch, asindicated by a block 510; and (5) providing a connection director unitcoupled with the first switch control unit and coupled with the secondswitch control unit, as indicated by a block 512.

Method 500 continues with the step of operating the connection directorunit to provide a first plurality of control signals and to provide asecond plurality of control signals, as indicated by a block 514. Method500 continues with the step of operating the first switch control unitand the second switch control unit to respond to the first plurality ofcontrol signals and the second plurality of control signals toselectively effect the first coupling and the second coupling, asindicated by a block 516.

Method 500 continues with the step of effecting at least one of thefirst coupling and the second coupling as an initial coupling and asubsequent continuing coupling; the initial coupling establishing agenerally time-dependent increasing current between the system and onepower supply of the first power supply and the second power supply; thecontinuing coupling establishing a substantially constant operatingcurrent between the system and the one power supply, as indicated by ablock 518. Method 500 terminates at an end locus 520.

It is to be understood that, while the detailed drawings and specificexamples given describe preferred embodiments of the invention, they arefor the purpose of illustration only, that the apparatus and method ofthe invention are not limited to the precise details and conditionsdisclosed and that various changes may be made therein without departingfrom the spirit of the invention which is defined by the followingclaims:

1. An apparatus for selecting a power supply in a dual supply system;the apparatus comprising: (a) a connection director unit; saidconnection director unit presenting a plurality of actuating signals;(b) a first switching unit coupled with said connection director unit,coupled with a first power supply and coupled with said system; saidfirst switching unit generating a first indicating signal to indicatewhether said first switch is closed; (c) a second switching unit coupledwith said connection director unit, coupled with a second power supplyand coupled with said system; said second switching unit generating asecond indicating signal to indicate whether said second switch isclosed; (d) said first switching unit being coupled with said secondswitching unit; said first switching unit responding to a first signalcombination of said second indicating signal and at least one firstactuating signal of said plurality of actuating signals for effecting afirst coupling between said system and said first power supply; and (e)said second switching unit being coupled with said first switching unit;said second switching unit responding to a second signal combination ofsaid first indicating signal and at least one second actuating signal ofsaid plurality of actuating signals for effecting a second couplingbetween said system and a second power supply; said at least one secondactuating signal including an initiating signal and an operating signal;said second switching unit responding to said initiating signal toestablish a generally time-dependent increasing current between saidsystem and said second power supply; said second switching unitresponding to said operating signal to establish a substantiallyconstant operating current between said system and said second powersupply.
 2. An apparatus for selecting a power supply in a dual supplysystem as recited in claim 1 wherein said first switching unit respondsto said second predetermined signal combination to interrupt said firstcoupling and wherein said second switching unit responds to said firstpredetermined signal combination to interrupt said second coupling. 3.An apparatus for selecting a power supply in a dual supply system asrecited in claim 1 wherein said first switching unit includes abreak-before-make circuit; said break-before-make circuit ensuring thatsaid first switching unit interrupts said first coupling before saidsecond switching unit effects said second coupling when said connectiondirector unit presents said second predetermined signal combination. 4.An apparatus for selecting a power supply in a dual supply system asrecited in claim 1 wherein said first switching unit includes a timerunit coupled for receiving said at least one first actuating signal;said timer unit operating to assure said substantially constantoperating current between said system and said second power supplyoccurs no more than a predetermined time interval after said connectiondirector unit presents said second predetermined signal combination. 5.An apparatus for selecting a power supply in a dual supply system asrecited in claim 2 wherein said first switching unit includes abreak-before-make circuit; said break-before-make circuit ensuring thatsaid first switching unit interrupts said first coupling before saidsecond switching unit effects said second coupling when said connectiondirector unit presents said second predetermined signal combination. 6.An apparatus for selecting a power supply in a dual supply system asrecited in claim 2 wherein said first switching unit includes a timerunit coupled for receiving said at least one first actuating signal;said timer unit operating to assure said substantially constantoperating current between said system and said second power supplyoccurs no more than a predetermined time interval after said connectiondirector unit presents said second predetermined signal combination. 7.An apparatus for selecting a power supply in a dual supply system asrecited in claim 3 wherein said first switching unit includes a timerunit coupled for receiving said at least one first actuating signal;said timer unit operating to assure said substantially constantoperating current between said system and said second power supplyoccurs no more than a predetermined time interval after said connectiondirector unit presents said second predetermined signal combination. 8.An apparatus for selectively coupling a system with a first power supplyor a second power supply; the apparatus comprising: (a) a first switchfor switchingly effecting a first coupling of said system with saidfirst power supply; (b) a second switch for switchingly effecting asecond coupling of said system with said second power supply; (c) afirst switch control unit coupled with said first switch for controllingsaid first switch; (d) a second switch control unit coupled with saidsecond switch for controlling said second switch; (e) a connectiondirector unit coupled with said first switch control unit and coupledwith said second switch control unit; said connection director unitproviding a first plurality of control signals to effect said firstcoupling; said connection director unit providing a second plurality ofcontrol signals to effect said second coupling; at least one of saidfirst coupling and said second coupling being effected as an initialcoupling and a subsequent continuing coupling; said initial couplingestablishing a generally time-dependent increasing current between saidsystem and one power supply of said first power supply and said secondpower supply; said continuity coupling establishing a substantiallyconstant operating current between said system and said one powersupply.
 9. An apparatus for selectively coupling a system with a firstpower supply or a second power supply as recited in claim 8 wherein saidfirst switch control unit and said second switch control unit cooperateto interrupt said coupling of said system with one power supply of saidfirst power supply and said second power supply when the other powersupply of said first power supply and said second power supply iscoupled with said system.
 10. An apparatus for selectively coupling asystem with a first power supply or a second power supply as recited inclaim 8 wherein at least one switch control unit of said first switchcontrol unit and said second switch control unit includes abreak-before-make circuit coupled with the other switch control unit ofsaid first switch control unit and said second switch control unit; saidbreak-before-make circuit ensuring that at least one of said firstswitch and said second switch interrupts said first coupling beforeeffecting said second coupling and interrupts said second couplingbefore effecting said first coupling.
 11. An apparatus for selectivelycoupling a system with a first power supply or a second power supply asrecited in claim 8 wherein at least one switch control unit of saidfirst switch control unit and said second switch control unit includes atimer unit coupled with the other switch control unit of said firstswitch control unit and said second switch control unit; said timer unitoperating to ensure said continuity coupling is effected by said otherswitch control unit no more than a predetermined time interval aftersaid initial coupling is established by said other switch control unit.12. An apparatus for selectively coupling a system with a first powersupply or a second power supply as recited in claim 8 wherein at leastone switch control unit of said first switch control unit and saidsecond switch control unit includes a break-before-make circuit coupledwith the other switch control unit of said first switch control unit andsaid second switch control unit; said break-before-make circuit ensuringthat at least one of said first switch and said second switch interruptssaid first coupling before effecting said second coupling and interruptssaid second coupling before effecting said first coupling.
 13. Anapparatus for selectively coupling a system with a first power supply ora second power supply as recited in claim 9 wherein at least one switchcontrol unit of said first switch control unit and said second switchcontrol unit includes a timer unit coupled with the other switch controlunit of said first switch control unit and said second switch controlunit; said timer unit operating to ensure said continuity coupling iseffected by said other switch control unit no more than a predeterminedtime interval after said initial coupling is established by said otherswitch control unit.
 14. An apparatus for selectively coupling a systemwith a first power supply or a second power supply as recited in claim10 wherein at least one switch control unit of said first switch controlunit and said second switch control unit includes a timer unit coupledwith the other switch control unit of said first switch control unit andsaid second switch control unit; said timer unit operating to ensuresaid continuity coupling is effected by said other switch control unitno more than a predetermined time interval after said initial couplingis established by said other switch control unit.
 15. A method forselectively coupling a system with a first power supply or a secondpower supply; the method comprising the steps of: (a) in no particularorder: (1) providing a first switch connected for switchingly effectinga first coupling of said system with said first power supply; (2)providing a second switch connected for switchingly effecting a secondcoupling of said system with said second power supply; (3) providing afirst switch control unit coupled with said first switch for controllingsaid first switch; (4) providing a second switch control unit coupledwith said second switch for controlling said second switch; and (5)providing a connection director unit coupled with said first switchcontrol unit and coupled with said second switch control unit; (b)operating said connection director unit to provide a first plurality ofcontrol signals and to provide a second plurality of control signals;(c) operating said first switch control unit and said second switchcontrol unit to respond to said first plurality of control signals andsaid second plurality of control signals to selectively effect saidfirst coupling and said second coupling; and (d) effecting at least oneof said first coupling and said second coupling as an initial couplingand a subsequent continuing coupling; said initial coupling establishinga generally time-dependent increasing current between said system andone power supply of said first power supply and said second powersupply; said continuing coupling establishing a substantially constantoperating current between said system and said one power supply.
 16. Amethod for selectively coupling a system with a first power supply or asecond power supply as recited in claim 15 wherein said first switchcontrol unit and said second switch control unit cooperate to interruptsaid coupling of said system with one power supply of said first powersupply and said second power supply when the other power supply of saidfirst power supply and said second power supply is coupled with saidsystem.
 17. A method for selectively coupling a system with a firstpower supply or a second power supply as recited in claim 15 wherein atleast one switch control unit of said first switch control unit and saidsecond switch control unit includes a break-before-make circuit coupledwith the other switch control unit of said first switch control unit andsaid second switch control unit; said break-before-make circuit ensuringthat at least one of said first switch and said second switch interruptssaid first coupling before effecting said second coupling and interruptssaid second coupling before effecting said first coupling.
 18. A methodfor selectively coupling a system with a first power supply or a secondpower supply as recited in claim 15 wherein at least one switch controlunit of said first switch control unit and said second switch controlunit includes a timer unit coupled with the other switch control unit ofsaid first switch control unit and said second switch control unit; saidtimer unit operating to ensure said continuity coupling is effected bysaid other switch control unit no more than a predetermined timeinterval after said initial coupling is established by said other switchcontrol unit.
 19. A method for selectively coupling a system with afirst power supply or a second power supply as recited in claim 15wherein at least one switch control unit of said first switch controlunit and said second switch control unit includes a break-before-makecircuit coupled with the other switch control unit of said first switchcontrol unit and said second switch control unit; said break-before-makecircuit ensuring that at least one of said first switch and said secondswitch interrupts said first coupling before effecting said secondcoupling and interrupts said second coupling before effecting said firstcoupling.
 20. A method for selectively coupling a system with a firstpower supply or a second power supply as recited in claim 16 wherein atleast one switch control unit of said first switch control unit and saidsecond switch control unit includes a timer unit coupled with the otherswitch control unit of said first switch control unit and said secondswitch control unit; said timer unit operating to ensure said continuitycoupling is effected by said other switch control unit no more than apredetermined time interval after said initial coupling is establishedby said other switch control unit.